Chalk Group Lithostratigraphy: East Anglia - Upper Chalk

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The thickest Upper Chalk in the UK occurs in north Norfolk, the eastward regional dip preserving in the eastern part of this county horizons that have been removed by Palaeogene erosion elsewhere in East Anglia. In the southern part of East Anglia, around Saffron Waldon, the preserved thickness of Upper Chalk is c. 77m (White, 1932), and further north, around Bury St Edmunds and Diss, the thickness of the Upper Chalk is c. 125 m and c. 220 m respectively. There is an increase in the eastward formational dip of the Upper Chalk in the eastern part of north Norfolk (Peake & Hancock, 1970), and here the Upper Chalk thickens dramatically to +420 m in the Trunch Borehole [TG 2933 3455] (Arthurton et al., 1994), with still younger stratigraphical horizons preserved in a series of glacially disturbed chalk rafts on the Norfolk coast at Trimmingham and Sidestrand.

The base of the Upper Chalk has been variously defined in East Anglia. The traditional marker for the base of the Upper Chalk in the adjacent Chilterns region, the Chalk Rock, was identified in a some previous surveys of the region, for example around Cambridge (Worssam & Taylor, 1968) and in the original survey of the Bury St. Edmunds district (Bristow, 1990). No Chalk Rock occurs in the Saffron Waldron district to the south (White, 1932), where the base of the Upper Chalk appears to have been inferred using biozonal criteria. The Chalk Rock of the Bury St. Edmunds district, and probably of most adjacent regions in East Anglia, comprises the highest hardground of Bromley & Gale's (1982) Chalk Rock 'Formation', named the Hitchwood Hardground (Mortimore & Wood, 1986), the base of which is stratigraphically higher than the base of the Chalk Rock of the Chilterns region where this bed was first used to trace the base of the Upper Chalk. Furthermore, the isolated descriptions of Chalk Rock in East Anglia suggest that it can only be identified in exposures, and is not feature-forming. Thus, in the resurvey of the Bury St. Edmunds district (Bristow, 1990) and the area around Diss (Mathers et al., 1993), the base of the Upper Chalk was lowered slightly to coincide with a feature-forming flint-rich interval, named the Brandon Flint Series. The latter follows traditional practice in the North Downs, where in the absence of the Chalk Rock, the base of the Upper Chalk was placed at the base of the 'Basal Complex', a succession of conspicuous flints and marls that is correlative with the Brandon Flint Series (Mortimore & Wood, 1986).

In contrast to the typically hard, nodular chalk that usually characterises the basal and lower parts of the Upper Chalk in the Southern Region, the correlative interval in parts of East Anglia (eg. Saffron Waldon, Cambridge and Bury St. Edmunds districts) is relatively weak to non-nodular, soft, smooth-textured or blocky chalk with nodular and tabular flints (Bristow, 1990; Worssam & Taylor, 1968; White, 1932). Bristow (1990) also notes the general absence of hardgrounds in the lower part of the Upper Chalk in the Bury St Edmunds district, in contrast with correlative horizons in the Southern Region. An exception to this is the locally indurated chalk that forms the Chalk Rock (see above), and a locally occurring bed of hard limestone capped by a hardground slightly higher in the succession, named the Top Rock. However, in north Norfolk, the basal Upper Chalk might be more indurated. At Bircham Newton [TF 7650 3705], Jukes-Browne & Hill (1904) recorded hard, white, nodular chalk with regularly developed tabular flints, and larger Paramoudra-like flints.

Much of the remainder of the Upper Chalk is poorly understood in East Anglia, merely comprising flinty, white chalk, with local shell-rich and coarse-grained bioclastic intervals, and rare marls. Flints, particularly tabular flints, are locally very common (White, 1932), but are notably less frequent in the U. socialis Zone and M. testudinarius Zone (Peake & Hancock, 1970; Mathers et al., 1993).

In the youngest part of the Chalk Group of north Norfolk (highest Campanian and Lower Maastrichtian) a number of subdivisions have been recognised. Those in the Campanian, named in ascending order: Pre-Weybourne Chalk, Weybourne Chalk, Beeston Chalk and Paramoudra Chalk (Wood, 1988), being recognised using lithological and/or faunal characteristics. The Maastrichtian Chalk succession, named in ascending order: Sidestrand Chalk, Trimingham Sponge Beds, Little Marl Point Chalk and Beacon Hill Grey Chalk (Johansen & Surlyk, 1990), are recognised mainly on lithological features. This youngest part of the succession contains intervals with large, vertically elongated flint cylinders (Paramoudra flints), ring-shaped flints, shell-rich bioclastic chalk and spongiferous chalk with hardgrounds and erosion surfaces.

Macrofossil Biozonation: S. plana Zone, M. cortestudinarium Zone, M. coranguinum Zone, U. socialis Zone, M. testudinarius Zone, O. pilula Zone, G. quadrata Zone, B. mucronata Zone s.l., B. lanceolata Zone s.l., B. occidentalis Zone

Correlation: see Correlation with other UK Chalk Group successions

References

ARTHURTON, R S, BOOTH, S J, MORIGI, A N, ABBOTT, M A W & WOOD, C J. 1994. Geology of the country around Great Yarmouth. Memoir of the British Geological Survey.

BRISTOW C R. 1990. Geology of the country around Bury St. Edmunds. Memoir of the British Geological Survey.

BROMLEY, R G & GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273 - 306.

JOHANSEN, M B & SURLYK, F.1990. Brachiopods and the stratigraphy of the Upper Campanian and Lower Maastrichtian Chalk of Norfolk, England. Palaeontology, Vol. 33(4), 823-872.

JUKES-BROWNE, A J & HILL, W.1904. The Cretaceous rocks of Britain. Vol. 3 - The Upper Chalk of England. Memoir of the Geological Survey of the United Kingdom.

MATHERS, S J, HORTON, A & BRISTOW, C R. 1993. The geology of the country around Diss. Memoir of the British Geological Survey.

MORTIMORE, R N & WOOD, C J.1986. The distribution of flint in the English Chalk, with particular reference to the 'Brandon Flint Series' and the high Turonian flint maximum. In SIEVEKING, G de C & HART, M B. The scientific study of flint and chert. Cambridge University Press, Cambridge.

PEAKE, N B & HANCOCK, J M. 1970. The Upper Cretaceous of Norfolk [reprinted with corrigenda and addenda] In LARWOOD, G P & FUNNELL, B M (eds.), The Geology of Norfolk. London & Ashford.

WHITE, H J O.1932. The Geology of the country near Saffron Walden. Memoir of the Geological Survey England and Wales (Sheet 205).

WOOD, C J. 1988. The stratigraphy of the Chalk of Norwich. Bulletin of the Geological Society of Norfolk, No. 38, 3- 120.

WORSSAM, B C & TAYLOR, J H. 1969. Geology of the country around Cambridge. Memoir of the British Geological Survey.

See: Chalk Rock, Top Rock, Brandon Flint Series, Basal Complex (North Downs), marl, hardground